Field of the Invention
This invention relates to piles and anchorages. More particularly this invention relates to piles of the kind used for structural support purposes and anchorages of the tie-back kind or of the kind embedded in a sea floor, for example, used for holding floating or semi-submerged structures in position in the sea. Structural support piles and anchorages of this kind have similar requirements in that their firm location within the ground is a prime consideration. For convenience hereafter reference is made only to piles and piling members and portions but it is to be understood that this expression is used generically and includes anchorages of the kind just mentioned and corresponding parts thereof except where the context forbids.
It is known to use steel members of `H` section as piling structures. The advantages of such piles are that they are light and robust to handle; they are easily cut down or extended in length; that they can carry high compressive and tensile loads and high driving stresses; that small volumes of displacement are involved so that they are easy to drive to great depth and through hard intermediate strata; that they are easy to drive in closely spaced groups; and that they involve minimal disturbance of surrounding ground.
It is also known to use large displacement solid or hollow driven piles made of various materials such as steel, concrete and timber. The advantages of such piles is that they give higher end bearing and shaft resistance than `H` Section piles and the cost of materials may be less.
Again it is known to use bored piles whereby a hole is bored in the ground and subsequently filled with concrete or grout. Such piles have often proved economic and have the advantage that their installation induces little or no vibration in the surrounding ground.
Despite the above mentioned advantages, `H` Section piles do have severe disadvantages in that their small cross sectional area minimizes both end bearing resistance and shaft resistance. It has already been proposed to overcome the above mentioned disadvantages by driving the piles to considerably extended depths so as to increase to satisfactory levels the shaft resistance, welding "wings" onto the sides of the `H` Section piling member, and by welding wings and plates onto the `H` Section member near its lower end. Whilst all of these proposals might well increase the end bearing resistance of the pile, they all substantially reduce the positive advantages of the `H` Section member of easy driving and add considerably to the pile driving operation costs.
Again all driven piles cause severe disturbance to the soil around the pile which may be detrimental to its load carrying capacity. Thus, when used in clay soils orientation of clay particles down the side of the pile gives rise to low residual angles of friction. Yet again, in granular soil, a critical or limiting depth of penetration is reached early on below which friction per unit pile length ceases to increase because of an arching phenomena of the juxtaposed material. Again in strata formed of weakly cemented material such as chalk or carbonated sand, shaft resistance is exceptionally low due to grain crushing at the shaft to ground interface and subsequent arch around the pile. Finally, it has been noted that "whipping" of the pile during driving can significantly reduce the shaft friction.
Again for bored piles there are a number of disadvantages such as deterioration of the sides of the borehole, stress relaxation of the ground around the pile shaft, difficulties of ensuring stability of the borehole before or during concreting or grouting, ensuring integrity of the pile during its formation.